Torsional transmission element with elastic response

ABSTRACT

A torsional spring that can be used as a joint adapted to transmit at torsion actuation with elastic response. The torsional spring relates to a compact torsional elastic element, provided with a linear torque characteristic which is also not affected by the direction of rotation. The torsional spring is capable of attaining a high transmissible torque relative to its weight and overall dimension.

TECHNICAL FIELD OF THE INVENTION

The work leading to this invention has received funding from theEuropean Union Seventh Framework Programme FP7/2007-2013, within theframework of the CYBERLEGs Project, grant agreement n° 287894.

The present invention regards a new configuration of a torsional springthat can be used in particular as a link adapted to transmit a torsionactuation with elastic response.

BACKGROUND OF THE INVENTION

In the field of robotics, and in particular in the field of wearablerobotics, the use of elastic actuators is frequent (see, for example,Veneman et al., “A Series Elastic-and Bowden-Cable-Based ActuationSystem for Use as Torque Actuator in Exoskeleton-Type Robots”, TheInternational Journal of Robotics Research 2006 25: 261; Pratt et al.,“The RoboKnee: An Exoskeleton for Enhancing Strength and EnduranceDuring Walking”, Proceedings of the 2004 IEEE International Conferenceon Robotics & Automation New Orleans, April 2004; Torres-Jara et. al, “Asimple and scalable force actuator”, Proceedings of 35th InternationalSymposium on Robotics, Paris, France, 2004; and Paine et al., “A NewPrismatic Series Elastic Actuator with Compact Size and HighPerformance”), wherein an elastic element is arranged between theactuator and the actuated mechanical element (see, for example, Pratt etal., “Series elastic actuators” in Proc. IEEE Intl. Conf. Intell. RobotsSyst., Pittsburgh, Pa., 1995, pp. 339-406). For example, as described inU.S. Pat. No. 5,910,720, there are several reasons behind the use ofthis type of actuation especially in robotics. It definitely implies aseries of advantages that can be summarised in the following points:

-   -   low exit impedance on the entire frequency spectrum;    -   possibility of controlling the exit impedance through software;    -   reducing the energy consumption;    -   high force/mass ratio;    -   high power/mass ratio;    -   inherent compliance in case of impact.

These types of actuators may be linear or rotary. Both types ofactuators may be implemented with elastic linear or torsional elements(generally springs or assembled devices comprising springs).

One of the criticalities when it comes to implementing this type ofactuators lies in the choice and construction or the elastic element.

The main specifications, which vary as a function of the application theactuators are used for, characterizing an elastic element to be used inan elastic actuators are:

-   -   Rigidity;    -   Maximum admissible load;    -   Admissible rotation or displacement;    -   Weight;    -   Overall dimension (shape).

With reference to the case of rotary actuators, whose field is a morespecific object of the present invention, and thus elastic elements inwhich the transmission of a torsional stress is carried out, the priorart provides for various embodiments.

Generally speaking, a torsional elastic element may be obtained by usingone of the following elements:

-   -   Wire helical torsional spring;    -   Machined (from an integral block) helical torsional spring;    -   Spiral torsional spring;    -   Mechanism which converts the linear spring action in a torsional        response;    -   Torsional response custom element;    -   The use of wire helical torsional springs implies the following        problems:    -   Low rigidity with respect to the requirements set by robotic        applications;    -   Difficulty of interfacing with the elements in series therewith.        The torque is transmitted through contact between an element and        the wire of the spring which, in case of high deformations,        slides on the surface of contact with the element;    -   Difficulty to obtain the bi-directionality of the response. In        order to obtain an element capable of worling in both directions        of rotation it is necessary to create a mechanism provided with        at least two springs with ensuing increase of complexity, mass        and dimensions;    -   Contact between coils during motion;    -   Remarkable overall bulk caused by the presence of spring lever        arms.

The machined springs used as torsional springs overcome some of theaforementioned drawbacks (see, for example, Helical Products Company,Inc. http://www.heli-cal.com). Specifically, they are metal cylinders inwhich there is formed a helical recess with one or more principles, sothat the cylinder takes on a helical shape.

One of the main advantages of this type of springs lies in thepossibility of providing the ends thereof (to become interface areaswith the elements to which they are fixed) with different shapes andwith fixing systems which allow forming various couplings (threadedholes, threaded ends, notched profiles etc.).

However, alike the wire springs these springs have a preferentialdirection of rotation and this makes them not suitable for use inapplications in which there is expected the application of torques inboth directions of rotation and it is required an identical torsionalresponse in both directions.

Also the use of spiral springs allows overcoming some drawbacks of thewire helical springs but there remains the impossibility to obtain atwo-directional response without using more than one spring and aconnection mechanism.

By using linear springs in an assembled device, which converts thelinear response thereof into a torsional output response, there can beobtained a two-directional response with desired rigidity andtransmissible torque characteristics. The drawbacks related to this typeof solution mainly lie in the large overall dimension required for theimplementation of the entire assembly.

Among known examples of torsional springs formed starting from asuitably machined metal element, with the aim of conferring the desiredproperties to the element, the one disclosed in Lagoda et al., “Designof an electric Series Elastic Actuated Joint for robotic gaitrehabilitation training” Proceedings of the 2010 3rd IEEE RAS EMBSInternational Conference on Biomedical Robotics and Biomechatronics, TheUniversity of Tokyo, Tokyo, Japan, Sep. 26-29, 2010, is an elasticactuator used in walking rehabilitation. The elastic element used in theactuator is obtained from a plate-like steel body in which there areformed two spiral recesses. The element reveals some problems inconnection with hysteresis, the contact between the coils that limitsthe applicable load and the relatively high difference between therigidity simulated with FEM analysis and actual rigidity.

An embodiment analogous to the one described above is in Wang, Shiqian,et al. “Efficient Lightweight Series Elastic Actuation for anExoskeleton Joint”. The shape of the elastic element is generallysimilar to the previous one though with increased torsional rigidity andthe attempt to overcome the problems of hysteresis and contact betweenturns. Stienen et al., “Design of a rotational hydro-elastic actuatorfor a powered exoskeleton for upper-limb rehabilitation,” IEEE Trans.Biomed. Eng., vol. 57, no. 3, pp. 728-735, March 2010, discloses aspring similar to the previous ones used in a hydro-elastic actuator forthe rehabilitation of an upper limb.

Another type of torsional spring obtained by machining a steel elementis described in Sergi et. al, “Design and Characterization of a CompactRotary Series Elastic Actuator for Knee Assistance During OvergroundWalking”, in Proc. IEEE Int. Conf. on Biomed. Rob. and Biomech., pp.1931-1936, 2012; in this case, a metal disc is excavated so as to obtainspokes in the shape of laminar coils which join a hub and an externalrim. Being disc-shaped, this element has a high diameter/height ratio.The maximum torque applicable is limited by the occurrence of contactbetween the coils.

Patent publication WO2008US61560 discloses a torsional element in whichthe elastic response is obtained by joining two parallel flanges withS-shaped elements. The elements for joining the two flanges are barsfolded and fixed to the elements for input and output of the torque inthe system. According to a simplified variant, shown in US20070698811,the connection between the flanges for the input and output of thetorque is obtained by using straight bars and not S-shaped ones. Theseare complex systems which require the assembly of a plurality of partshence implying various complications. The connections between theelements must be stable and free of play, so as to avoid a torquetransfer mode (angle/torque characteristic) not repeatable or differentfrom the desired one, and the occurrence of unexpected stresspotentially causing damage to the structure. In addition, the machiningof all elements should be extremely accurate so as to avoid theoccurrence of residue stresses after assembly which may modify thecharacteristic of the elastic element or reduce the resistance thereof.

Again, the aforementioned patent U.S. Pat. No. 5,910,720 shows atorsional spring obtained with an element having cross-shaped sections,and thus the use of plates as the basic torque transfer element.However, the cross represents a configuration of plates workingsubstantially “in parallel”, hence requiring, with the aim of obtaininga high transmissible torque/rigidity ratio, i.e. high transmissibletorque but limited rigidity (high deformability), the use of very thinplates (excessive stresses) or the increase in the longitudinaldimension of the object (excessive overall dimension).

Another known torsional response element has a torsional propertyobtained by forming, on a cylindrical ring in charge of the transmissionof the torque between an input element and an output element, a seriesof recesses that with a radial development define a plurality ofsegments, in turn developing according to radial planes, that is passingthorough the torsion axis. The whole device is realized in multipleparts that require a rigid and precise mutual connection. Moreover,obtaining the recesses in the radial directions requires to carry out anumber of cuts on the ring body, with a resulting constructivecomplication deriving from the necessity of changing over and over themutual placement between the body and the cutting tool. Furthermore,being the transmissive segments arranged in a ring-like body, theyremain displaced from the torsion axis, and thus they tend to becomedeformed in flexion, realizing an unsatisfactory ratio betweentransmissible torque and rigidity.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a compact torsionalelastic element, provided with a linear angle/torque characteristicwhich is also not affected by the direction of rotation, and that issimple to obtain, allows avoiding unwanted contact between its parts,has high capacity of interfacing with the elements to which it should beconnected, and is capable of attaining a high transmissible torquerelative to its weight and overall dimension.

These and other objects are accomplished by the torsional springaccording to the present invention, whose essential characteristics aredefined in the first of the attached claims. Further importantcharacteristics are defined by the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the torsional spring according tothe present invention shall be more apparent from the followingdescription of its embodiments provided solely by way of non-limitingexample with reference to the attached drawings, wherein:

FIG. 1 shows in axonometric view a conceptual structure of theinvention, which also represents a first and elementary embodiment;

FIG. 2 shows in axonometric view a second embodiment of the springaccording to the invention;

FIG. 3 is a side view of the spring of FIG. 2;

FIG. 4 is a sectional view of the spring taken along lines IV-IV of FIG.3; and

FIG. 5 is a sectional view of the spring taken along lines V-V of FIG.4.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the above figures, the elastic element according tothe invention is represented by a typically metal element which, due tosuitable machining, takes the characteristics of a torsional spring withthe desired rigidity and transmissible torque properties. Typically,starting from a solid block integral body, material is taken away so asto obtain a distribution of recesses, as described hereinafter.

In particular, referring now only to FIG. 1, a body 1 is in this caseparallelepiped-shaped, and has an axis X which centrally traverses thebody and which represents the centre axis of the torque or moment oftorsion which is intended to be transmitted (between actuator and load)by making use of the body itself. Such axis shall be physically definedas the line that joins two opposite attachments formed in the body orassociated therewith, not shown in this case, for connecting the body tothe actuator and to the actuated element, in turn not represented. Theconnection is carried out with the assistance of suitable bearings forisolating the body from any stress different from the torsional stress.The opposite sides corresponding to the sides of attachment areindicated with 1 a and 1 b.

According to the invention, the body 1 has recesses 2, 3 mainlydeveloping according to planes A with a parallelism relationship withrespect to the axis X, which in case the body is parallelepiped-shapedmay also be identified as parallel, besided to each other, also to thelateral faces of the parallelepiped. Preferably the distribution ofrecesses 2, 3 has a symmetry with respect to a plane orthogonal to theplanes A and passing through the axis X, and also a substantialsymmetry, from a dimensional point of view, with respect to a planeorthogonal to the planes A and crossing an intermediate point of thebody 1 (intermediate point intended with respect to the elongation alongthe torsion axis X).

Still according to a preferred solution, the recesses 2, 3 are throughrecesses, i.e. open on three sides where the closure side is, for thevarious parallel and superimposed recesses, from time to time andalternatively on either of the attachment surfaces 1 a and 1 b. Therecesses 2 are those of the type closed on the side 1 a, while therecesses 3 are of the type closed on the opposite side. In practice, dueto such configuration, the body 1 takes a serpentine developing shape,where sectioned orthogonal to the planes A and to the two faces 1 a, 1b, determined by plate-like segments 11 spaced by the thickness(measurement in the direction orthogonal to the planes A) of therecesses 2, 3. The junctions between the segments 11, that close therecesses on the sides 1 a, 1 b, are indicated with 12.

The resulting torsional behaviour is definitely similar to that of aplate with length equivalent to the sum of the lengths (size along X) ofthe various plate-like segments 11, but with the difference lying in thefact that the junctions 12 stiffen the structure with respect to theequivalent plate. A further difference in the response lies in the factthat in a hypothetical equivalent spring made using a single plate, thematerial is entirely astride the torsion axis, while in case of theserpentine shape the most peripheral plate-like segments are in a stateof non-purely torsional stress and they cause a more rigid globalresponse.

The compactness of the obtained structure, especially in axialdirection, is then as such a substantial advantage, the desired rigiditybeing optimisable by acting on various geometrical parameters such as inparticular the thickness of the segments 11, the height-wise dimensionsthereof, i.e. their size over the planes A orthogonally to X, and thelength-wise dimensions, i.e. the size over the planes A parallel to X.In particular, the rigidity of the element increases as the thicknessand height increases and reduces as the axial length of the entirestructure increases.

Obviously, a fundamental variant for obtaining the desiredcharacteristics lies in the materials used; the most suitable materialsare the metals generally used in mechanical constructions. They includesteel, aluminium alloys and titanium alloys. Primarily, there may beidentified in the Young's modulus of the selected material, thefundamental parameter for obtaining the desired rigidity characteristicsof the element. Besides the desired rigidity, the selection of thematerial to be used directly follows the amount of load that the springshould be capable of bearing and the degree of dimensional compactnessto be obtained. The elbow junctions 12 between the plate-like segments11 represent the areas of concentration of the tensions; the higher theresistance of the material, the narrower the recess between the twoconsecutive segments shall be, and the smaller the overall thickness ofthe element (as mentioned, the thickness being intended as the dimensionin the direction orthogonal to the planes A).

With reference to FIGS. 2 to 5, a second embodiment of the torsionalspring according to the invention provides for a cylindrical body 101instead of a parallelepiped one. The serpentine configuration withrecesses 102, 103 and plate-like segments 111 is however entirelysimilar to the previous one, also in this case comprising plate-likesegments parallel to each other and to axis X, save for the fact thatthe segments clearly do not have a uniform height like in the previouscase, but they reduce in respect of this size as they move away from theaxis X, due to the circular curvature of the body.

Moreover, in this embodiment, the thickness of the segments 111 is notconstant but (FIGS. 4 and 5) it increases progressively for the mostperipheral segments, with the aim of harmonising the tensions in thematerial due to the torsion. Indeed, the peripheral segments must beargreater specific stresses, due to the nature of the torsion stress, andas also mentioned above they have a smaller height with respect to thecentral ones.

This embodiment further comprises two flanged attachments 104, 105,respectively on the sides 101 a, 101 b, in turn obtained in the samesolid block body due to two crosswise cuts 106, 107 which—on thecylinder 101—practically separate the portion of the actual serpentinefrom the ends of the cylinder, forming two disc-shaped portions whichare then suitably machined to make them adapted to the requiredmechanical junctions. The crosswise cuts stop before entirely cuttingthe cylinder section, leaving respective connection bridges 120, 122between the portion of the serpentine and the discs. Advantageously,such bridges are in diametrically opposite positions.

Specifically, the recesses and the notches can be obtained through wireelectro-erosion on a maraging steel bar (Böhler W720, Young's module:193 GPa, yield stress of 1815 MPa). Compatibly with the dimensions ofthe recesses and with the dimension of the section of the entire elementthe machining can be obtained by chip removal machining. A spring thusobtained, having an angle/torque characteristic that is linear and freeof hysteresis, is adapted to attain a torsional rigidity of 100 N·m·rad¹and transmissible torque of at least 30 N·m.

The ends of the serpentine element may be formed in various ways adaptedto allow interfacing the deformable element with the various types ofmechanical elements, obviously according to what can be implemented by aman skilled in the art. Junctions/fixing elements that can be used mayinclude flanged junctions with screws, shaft/hub junctions, notchedprofiles, keys, tabs, radial pins, spline elements etc.

Thus, the present invention allows overcoming the difficulties ofimplementing a torsional elastic element that is compact, robust andrelatively light, and which simultaneously allow transmitting hightorques with high deformability, thanks in particular to the arrangementwith recesses (and segments) parallel to each other and to the torquecenter axis. The realization in a single piece avoids any unwantedcontact between movable parts during use, and the element is also easyto interface with the parts to which it should be connected.

Other advantages that can arise from the use of the torsional springaccording to the invention comprise:

-   -   easy parametrisation of the dimensions with the aim of obtaining        springs with the desired characteristics;    -   possibility of designing the ends of the spring to use various        fixing methods between the spring and the elements to be        connected thereto;    -   possibility of obtaining the machining directly on a shaft thus        making it inherently elastic.

The spring is mainly applied in the field of robotics and in particularwearable robotics. The dimensions, the rigidity and transmissible torquecharacteristics thereof and its high capacity to interface with theother elements, make the present invention useful for obtaining elasticactuators for wearable robots and for robots in general. Actually, inthese applications it is fundamental to use actuators with limitedweights and overall dimensions while satisfying the need of transmittingrelatively high torques and forces. The elastic actuator according tothe invention, complete with all the elements, may be assembled directlyon the robot. The applicability of the spring is not however limited tothe field of robotics but it can also be extended to all fields thatrequire the use of torsional springs with given rigidity andtransmissible torque characteristics.

The present invention has been described with reference to preferredembodiments thereof. However, there can be provided other embodiments ofthe same inventive concept, falling within the scope of protection ofthe following claims.

The invention claimed is:
 1. A transmission element having a center axisand comprising: a block body; a plurality of through recesses formed inthe block body, the through recesses being generally parallel andsuperimposed mainly according to recess planes with a parallelrelationship with respect to the center axis, the through recessesspacing a plurality of plate-like segments of the block body, theplate-like segments being parallel with each other and alternatelyconnected by junctions on a first side and a second side of the blockbody; wherein the through recesses are alternately closed by thejunctions on the first and second sides of the block body, the first andsecond sides of the block body being opposite each other along thecenter axis, the plurality of through recesses defining a serpentineportion comprising the plurality of plate-like segments spaced by athickness of the through recesses; wherein the transmission element isarranged for providing an elastic response for transmission of atorsional stress between an actuating element and an actuated element;further comprising attachments for the actuating element and theactuated element formed in, or associated with, the block body inopposed positions aligned along a center axis of the torsion stress;wherein the attachments to the actuating element and the actuatedelement correspond respectively to the first and second sides of theblock body, the serpentine portion having ends connected respectivelywith the attachments to the actuating element and the actuated elementvia connection bridges at opposite sides of the center axis of thetransmission element; wherein the block body is cylindrically shapedfrom a metal material; wherein the attachments to the actuating elementand the actuated element are disc-shaped as a result of respectivecrosswise cuts formed in the cylindrically-shaped block body to separatethe serpentine portion from axial ends of the cylindrically-shaped blockbody, the crosswise cuts leaving the respective connection bridgesconnecting the serpentine portion to the disc-shaped attachments;wherein a thickness of the plate-like segments progressively increasesas the plate-like segments are positioned farther away from the centeraxis.
 2. The transmission element of claim 1, wherein the connectionbridges are in diametrically opposed positions.
 3. The transmissionelement of claim 1, wherein the through recesses are formed throughcutting away material from an integral block.
 4. The transmissionelement of claim 1, wherein a thickness of the junctions is greater thana thickness of the plate-like segments.
 5. The transmission element ofclaim 1, wherein the junctions connecting the plate-like segments defineareas of concentration of tension such that the higher the resistance ofmaterial forming the junctions, the narrower the through recesses aredefined between adjacent plate-like segments.
 6. The transmissionelement of claim 1, wherein the plate-like segments have a uniformheight.
 7. The transmission element of claim 1, wherein the block bodyis a solid block integral body such that the through recesses aredefined by areas of material removed from the block body.
 8. Atransmission element having a center axis and comprising: a block body;a plurality of through recesses formed in the block body, the throughrecesses being generally parallel and superimposed mainly according torecess planes with a parallel relationship with respect to the centeraxis, the through recesses spacing a plurality of plate-like segments ofthe block body, the plate-like segments being parallel with each otherand alternately connected by junctions on a first side and a second sideof the block body; wherein the through recesses are alternately closedby the junctions on the first and second sides of the block body, thefirst and second sides of the block body being opposite each other alongthe center axis, the plurality of through recesses defining a serpentineportion comprising the plurality of plate-like segments spaced by athickness of the through recesses; first and second flanged attachmentson opposed first and second sides of the junctions; wherein the firstand second flanged attachments are spaced apart from the junctions byfirst and second crosswise cuts, respectively.
 9. The transmissionelement of claim 8, wherein the transmission element forms a cylinder.10. The transmission element of claim 9, wherein the first and secondcrosswise cuts extend short of extending through an entirety of thecylinder.